1. Field of the Invention
The present invention relates to a piezoelectric vibrator bonded with a piezoelectric vibrating piece at a lead terminal and its fabricating method, a piezoelectric oscillator, an electronic apparatus, and a radio wave timepiece.
2. Description of the Related Art
A piezoelectric vibrator is an electronic element indispensable for fabricating industrial products such as a timepiece, an oscillator, an electronic apparatus or the like, and is used as a time source, a timing source, or a reference source of a signal. As a package of a piezoelectric vibrator, a cylinder type package in a cylindrical shape is commonly used. A constitution of a piezoelectric vibrator of the cylinder type package will be explained in reference to the drawings.
FIG. 13 is a disassembled perspective view showing an inner structure of a piezoelectric vibrator of a cylinder type package, and FIG. 14 is a fabrication flowchart schematically showing the fabricating of the piezoelectric vibrator.
As shown by FIG. 13, a piezoelectric vibrator 6 of a cylinder type package is constituted by a structure in which a piezoelectric vibrating piece 4 is bonded to an inner lead 2 on an inner side of a stem 8 constituting a metal ring of an airtight terminal 1 including two pieces of metallic lead terminals. The piezoelectric vibrating piece 4 comprises a piezoelectric material of quartz or the like and is formed in a shape of a tuning fork by a photolithography technology. Exciting electrodes 4a are formed at surfaces of two pieces of vibrating arm portions of the piezoelectric vibrating piece 4 of the tuning fork type. A base end side of the piezoelectric vibrating piece 4 is formed with a mount electrode 7 communicating with the exciting electrode 4a. 
The piezoelectric vibrating piece 4 and the inner lead 2 are bonded by the mount electrode 7. The inner lead 2 is penetrated through hermetic glass at inside of a stem 8, and a portion thereof mounted on a board is referred to as an outer lead (designated by notation 3). The inner lead 2 and the outer lead 3 are generally referred to as lead terminal. An outer periphery of the stem 8 is covered with a sealing tube of a bottomed cylinder member in a cylindrical shape made of a metal, not illustrated, to cover the piezoelectric vibrating piece 4 of the tuning fork type to be subjected to airtight sealing in vacuum.
According to the piezoelectric vibrator of the cylinder type package constituted as described above, when a predetermined voltage is applied to the two pieces of the outer leads 3 as a drive voltage, a current flows from the inner lead 2 to the exciting electrode 4a by way of the mount electrode 7, and the piezoelectric vibrating piece 4 is oscillated by a predetermined frequency.
As shown by FIG. 14, steps of fabricating the piezoelectric vibrator 6 include a step of bonding the inner lead 2 and the piezoelectric vibrating piece 4, a frequency trimming step of achieving a predetermined frequency from the piezoelectric vibrating piece 4, and a sealing step of sealing the sealing tube of the bottomed cylinder member of a metal to the stem 8 of the airtight terminal 1, and among the steps, a heating step and a step in vacuum are included. In the respective fabricating steps, a carrying pallet is utilized as means for positioning the piezoelectric vibrator 6, and the piezoelectric vibrator 6 is fabricated.
FIG. 15 is an outline perspective view showing a carrying pallet for the piezoelectric vibrator of the background art and the piezoelectric vibrator positioned in the carrying pallet. In the respective fabricating steps, according to the airtight terminal 1 including the lead terminal, as shown by an enlarged view on a right upper side of FIG. 15, one end of the outer lead 3 is fixed by a fixing spring 19 bonded to a carrying pallet 18 of the piezoelectric vibrator 6. The carrying pallet 18 of the piezoelectric vibrator 6 fixing the outer lead 3 is positioned to a fabricating apparatus in the heating step, the step in vacuum and the like to carry out a bonding operation, a frequency trimming operation, a sealing operation or the like. In the step of bonding the inner lead 2 and the piezoelectric vibrating piece 4, the piezoelectric vibrating piece 4 is aligned to an aligning jig (not illustrated) constituting aligning means and the carrying pallet 18 fixing the outer lead 3 is positioned to the aligning jig. Thereby, the inner lead 2 and the piezoelectric vibrating piece 4 on the carrying pallet 18 are positioned.
There is known a method of bonding the inner lead 2 and the piezoelectric vibrating piece 4 by applying hot wind of nitrogen or the like and pertinently heated from above an end portion of the piezoelectric vibrating piece 4 and the inner lead 2 positioned to each other. In order to promote a wettability of solder, there is also known a method of adding excitation active species of a discharge gas to the hot wind such as nitrogen gas.
According to these methods, a previously plated solder of the inner lead 2 is melted by the hot wind, and the inner lead 2 and the piezoelectric vibrating piece 4 are bonded. Further, as the plating with the solder, there is commonly used a method of forming several hundreds thousands pieces of airtight terminals by an electrolytic barrel plating method, at a step of fabricating the airtight terminal and the solder is formed with a thickness of about 10 through 15 μm at a surface of the lead terminal and an outer peripheral portion of the stem.
In soldering, a flux is frequently used with an object of promoting a wettability of the solder by removing an oxide film formed naturally in air on a surface of a base material. However, in the case of the piezoelectric member, there is a case in which an electrode on a surface of the piezoelectric member is corroded by residue or splash of the flux to deteriorate a performance thereof and therefore, a nonflux method which does not use a flux has been adopted.
Accordingly, the oxide film or the contamination cannot completely be removed from the surface of the base material and therefore, depending on the state of the surface, the wettability of the solder is significantly varied, and there is a case of bringing about a failure in soldering. The acceptability of the performance of bonding the inner lead 2 and the piezoelectric vibrating piece 4 effects a significant influence on various properties, an accuracy in a frequency and reliability of the piezoelectric vibrator 6.
In order to avoid the failure in soldering owing to low wettability of the solder, there are methods grossly classified into two: a method of a countermeasure of elevating a heating temperature or the like and a method of mixing the excitation active species to the hot wind. A description will be given of respective problems with regard to the two countermeasure methods.
(Problem in a Case of Elevating a Heating Temperature)
According to a heating method by the hot wind of nitrogen or the like, when the piezoelectric vibrating piece comprises quartz, the hot wind is blown to quartz, and a temperature of the vibrating piece per se is excessively elevated. In this case, there is a concern that a strain of twining or the like is generated in a crystal of quartz, properties such as a crystal impedance value, a temperature coefficient and the like are considerably deteriorated. Particularly, at current time in which downsizing of the piezoelectric vibrating piece and the airtight terminal is rapidly progressed, caution is required for a heat capacity of a member to be combined so as not to a deteriorate the members in the midst of the step.
Describing in details, first, the heated hot wind is applied from above the end portion of the piezoelectric vibrating piece 4 and the inner lead 2, the solder of the inner lead 2 is melted to be bonded therewith, however, there is a case in which the heat capacity necessary for melting the solder is not satisfied and excellent bonding by melting the solder is not achieved.
That is, according to the method of heating the hot wind, the heat capacity received by the heated portion is proportional to a wind amount of the hot wind and an area of the heated portion and therefore, a temperature is elevated from a position having a large area of the heated portion. A diameter of the inner lead 2 plated with the solder is small and the heated area is small and therefore, a temperature rise thereof is small.
A large amount of the hot wind is needed in order to provide the heat capacity of the hot wind necessary for melting the solder of the inner lead 2. It is necessary to make the hot wind easy to pass through a surrounding of the heated portion therefor. However, when the hot wind is blocked by a portion of a jig, portion other than the bonding portion is also heated to be melted. As a result, an adverse influence is effected on the vibrating piece as described above. In mounting with a solder having a high melting point such as a heat resistant solder plating, a problem is particular posed thereby. Depending on a shape of the surrounding through which the hot wind passes, an inverse case may be also brought about in which the wind amount of the hot wind is restricted, the heat capacity necessary for melting the solder of the inner lead 2 is not obtained and thus unmelted bonding is present.
Furthermore, in the bonding step, in order to bond a plurality of the airtight terminals successively, a plurality of the airtight terminals and the vibrating pieces are positioned each other beforehand on the carrying pallet 18. The carrying pallet 18 is moved in the direction of long side of the pallet with a constant speed under the nozzle of the hot wind. Thereby, A plurality of the inner lead 2 and the piezoelectric vibrators piece 4 on the carrying pallet are bonded one after another. However, the method of blowing the hot wind to a moving object is liable to undergo a difference in the wind amount owing to a slight deviation in a nozzle angle, fluctuation of the atmosphere and there is a case in which excellent bonding by melting the solder is not achieved owing to an instability in a provided heat amount.
In this way, the heating method by the hot wind poses a problem in view of control of a supplied heat amount and the object to be supplied therewith.
(A Problem of a Method of Utilizing an Excitation Active Species)
In a related art, there is known a method of executing a mounting step by mixing an excitation active species to hot wind. For example, in order to generate an excitation active species, CF4 is selected as a discharge gas, and the excitation active species of fluorine generated in a discharge tube is mixed with a heated hot wind of nitrogen and is injected to a corresponding portion to be mounted. Thereby, a thin fluoride layer is formed on a solder plated layer of a surface of an inner lead and an electrode face to be mounted thereon, at the same time, the solder plated layer is melted and bonding excellent in a wettability can be realized.
However, CF4 gas selected as the discharge gas is a gas showing a green house effect and a gas which is preferably avoided to be used in a fabricating step. Further, a mounting step is a step of frequently repeating supply and take out of a pallet aligned with the piezoelectric vibrating piece 4 and the airtight terminal 1, and it is preferable that an apparatus per se is an apparatus opened to the atmosphere. In this respect, use of the excitation active species of CF4 gas causes to contaminate surrounding air, which is not preferable. Furthermore, since as described above, the hot wind is used on a basis thereof, a controllability is inferior in supplying necessary amounts of the hot wind and the excitation active species only to necessary portions of the piezoelectric vibrating piece 4 and the airtight terminal 1 which are downsized. Further, in operating the apparatus for a long period of time, the excitation active species is supplied also to the apparatus and jigs of the surrounding and therefore, it is necessary to stop the apparatus frequently to disassemble and clean the jigs to poses a problem in view of efficient production.
Next, a description will be given of other problem in the mounting step. A mechanical positioning accuracy of the inner lead 2 and the piezoelectric vibrating piece 4 effects a significant influence on various properties and the accuracy in the frequency and the reliability of the piezoelectric vibrator 6 in addition to the bonding performance and is ranked to be the most important for the performance of the piezoelectric vibrator 6.
Positioning of the inner lead 2 and the piezoelectric vibrating piece 4 is carried out by positioning the carrying pallet 18 to the aligning jig of the piezoelectric vibrating piece 4. However, the one end of the outer lead 3 is fixed by the fixing spring 19 mounted to the carrying pallet 18 and therefore, there is a case in which a positional accuracy necessary for positioning the inner lead 2 and the piezoelectric vibrating piece 4 is not achieved. It just so happens that a tip of vibrating arms of vibrating piece 4 touch an inner wall of the sealing tube 5. The oscillation becomes unstable by the touch. And the touch produces fine particles such as quartz or a conductive material peeled off from the inner wall of the sealing tube 5. When these particles stick to vibration piece 4, a slight frequency shift occurs and oscillation becomes unstable.